This invention is generally related to micro-electromechanical (MEM) switches and to a method of fabricating such structures, and more specifically, to a torsional MEM switch having the control electrodes perpendicular to the switching electrodes.
Switching operations are a fundamental part of many electrical, mechanical and electromechanical applications. MEM switches have drawn considerable interest over the last few years. Products using MEM switch technology are widespread in biomedical, aerospace, and communications systems.
While MEM switches have been manufactured utilizing torsion beams, as described, for instance, in U.S. Pat. No. 6,000,280 to Miller et al., these devices have been typically constructed with the control surfaces parallel to the switch surface. Moreover, the materials and processing have also not been compatible with CMOS fabrication techniques.
In order to gain a better understanding of the present invention, a conventional MEM switch will now be described with reference to FIG. 1, which shows a cross-section view of a MEM switch having one end of deformable beam 5 anchored on dielectric 2. The lowest level is made of dielectric material 1 consisting of conductive elements 3 and 4 which are used to connect or form the various electrical components of the device. The conductors referenced by numerals 3 and 6 are used to provide an operating voltage potential that causes the beam to deform. Conductor 4, which conducts a signal is, in turn, connected to the beam when the MEM switch is in operation. FIG. 2 shows a top-down view of the same conventional switch.
In a typical implementation of a prior art MEM switch, a contact beam 6, is formed by polysilicon over a dielectric layer made of, e.g., SiO2. The surrounding material is etched away leaving a raised structure attached to silicon beam 5. The contact beam 6 suspended above conductors 3 and 4 that were previously formed is preferably made of polysilicon. Subsequently, the device is subjected to electroless plating, usually of gold, that adheres to the polysilicon forming conductive elements 3, 4 and 6.
The switch operates by providing a voltage difference between contact beam 6 and electrode 3. This voltage generates an electrostatic attraction that brings beam 6 in contact with electrode 4, thus closing the switch. The twist imparted to the anchored beam 5 is used to restore contact 6 to its open position once the control voltage potential is dropped.
Generally, all conventional raised structures are characterized by extending over a very large area when it is compared to conventional semiconductor devices. This, in itself, makes them virtually impossible to integrate into the semiconductor chip fabrication process.
Accordingly, it is an object of the invention to provide a torsional MEM switch having its control electrodes substantially perpendicular to the switching electrodes.
It is another object to provide electrical isolation between the control signal and the switched signal by separating the contacts by a dielectric.
It is further an object to provide a torsional MEM switch with multiple controls for opening and closing the switch.
It is yet another object to provide MEM switches in a variety of multi-pole, multi-throw arrangements.
It is still another object to provide a MEM switch having a greatly reduced overall switching area needed to make good contact between the electrodes, while still maintaining good electrostatic control.
It is still a further object to provide a method of fabricating a MEM switch using manufacturing techniques that are compatible with those applicable to CMOS semiconductor devices.
In one aspect of the invention, the overall switch area of the MEM switch that is required to make good contact and provide the necessary electrostatic controls is greatly reduced by placing the controls in a direction perpendicular to the switch contacts. This not only moves the control surface area in a perpendicular direction, but it also shortens the length of the beam due to the added leverage gained in the perpendicular arrangement. The same leverage lowers the control voltage requirements since the spacing between the control electrodes can be reduced without downsizing the spacing between the signal electrodes.
In another aspect of the invention, the problem occurring in MEM switches known as stiction (i.e., the tendency for surfaces making contact to stick together and not release when the control voltage is dropped) is greatly reduced. The inventive MEM switch addresses this problem by having opposing control surfaces supply the necessary attraction in either direction, thus overcoming stiction.
In still another aspect of the invention, isolation of the control signal from the switching signal is provided. The inventive MEM switching device physically and electrically isolates the two conducting paths. It also provides an added isolation by significantly increasing the spacing between the signal electrodes by rearranging the various elements forming the MEM switch. The invention further provides for single and multiple pole devices.
In yet another aspect of the invention, there is provided a semiconductor MEM switch that includes: a conductive movable control electrode; an insulated semiconductor torsion beam attached to the movable control electrode, the insulated torsion beam and the movable control electrode being parallel to each other; and at least one movable contact attached to the insulated torsion beam, wherein the combination of the insulated torsion beam and the control electrode is perpendicular to the movable contact.